on 20 November 2008

Since the inception/provision of early osseo-integrated dental implants, this science has had to evolve continuously as increasingly more is known about how differences in implant length sizes, implant surfaces and materials can contribute to good long-term success and this coupled with functional and cosmetic considerations ensure that a clinician has to be better informed to keep abreast of such developments.

This article hopes to pay particular reference to the concept of crestal bone loss, which is a phenomenon associated after the placement of dental implants.

A criterion for implant success was proposed in 1986(1). Parameters here specifically related to the individual implant being immobile when tested clinically, a radiograph not demonstrating any evidence of peri-implant radioluscency, vertical bone loss being less than 0.2mm annually post-two months, the absence of chronic and acute symptoms of pain, infection, par aesthesia, neuropathies and finally in the context of the above parameters a success rate of 85% and 80% at five and 10 year observations.

Such criteria for success means that any given implant system can be validated in terms of scientific and clinical analysis.

The role of surface roughness on the biocompatibility of titanium implants has been discussed by various authors. There are two main mechanisms of bone-implant attachment: mechanical attachment and chemical attachment.

Roughness mainly improves mechanical attachment as it permits bone to grow through pores or features on the surface of the implant. On the other hand, the contribution of roughness to the improvement of osseointegration is not only mechanical. Features like surface tension and thus surface energy, can change the hydrophobic character of a surface. The ability a surface has to absorb organic molecules like proteins is directly related to biocompatibility. In this context, surface roughness has been found to positively influence cell response to titanium implants (2).

Some authors have also investigated the influence of surface roughness on bone attachment and concluded that rougher surfaces result in improved tissue responses to titanium implants (3, 4, 5). The success of osseointegrated implants is dependant on the establishment and maintenance of a direct structural and functional link between the surface of the load carrying implant and the surrounding bone.

Tissue integration of endo-osseous implants for osseo-integration is generally associated with the loss of vertical bone height during the first year after placement. (5) This usually begins at the crestal area of the cortical bone and can progress towards the apical area jeopardising the health of the implant itself.

Crestal bone loss being the most common cause of failure of implants in that osseointgeration has been achieved. The amount of bone loss reported during the first year can be 1.6mm. The observation of additional bone loss has lead to the acceptance criteria to evaluate different osseointegrated implant systems.

A mean annual vertical bone loss following the first year after placement of <0.2mm is generally accepted for successful treatment outcomes (7).

Several investigators have looked/attempted to minimise crestal bone loss by increasing the contact area of bone to implant surface and therefore reducing the stress at the cortical alveolar crest. Palmer et al 1997 (8) showed that by altering the diameter and/or length of the implant fixture or by altering its design and shape you increase the contact area of the bone to implant interface.

Increasing the rate of early endosseous integration is a critical goal to achieve improved implant success. Considerable attention has been given to the development and testing of implant surfaces since surface characteristics have been shown to have significant influence on the bone implant interface.

Subtractive methods such as blasting, acid etching have also been used to increase the surface area and to alter micro-topography or texture of the implant surface. Buser et al (1991)(9) demonstrated that textured implant surfaces that were sandblasted and acid-etched achieved a greater bone to implant contact than topographically titanium plasma sprayed surfaces.

Indeed, implant surfaces that were sandblasted and acid-etched performed better than implant surfaces that were either corundum blasted, sandblasted and acid pickled or plasma spray coated in relation to less marginal bone loss.

The implant abutment junction (IAJ) is said to contribute to the establishment of the biologic width and as such its location with respect to the bony crest that has been deemed to be of importance.

The biological width being the cumulative total of the sum of the epithelial attachment and the connective tissue attachment that is typically 2.03mm. The work of Hermann established that for differing depths of placement of the IAJ, the same 2.0mm of crestal bone loss is always established and as such, deeper placement of the implant will result in greater crestal bone loss, this occurring regardless of whether the implant was micro-textured or not, suggesting that the establishment of the biologic width outweighs the bone bonding power of such surfaces. A concept of platform-switching was described by Baumgarten (2005).

This utilises an abutment of smaller diameter than that of the implant platform and a divergent implant collar to allow the formation of the biological space partially on the uncovered part of the implant platform. This platform incorporates a coronal bevel that in turn mesialises the IAJ.This theory by decreasing the connective tissue attachment inflammatory processes, achieves and enhances crestal bone preservation which in turn decreases the amount of peri-implant cervical bone resorption and potentially allows for greater soft tissue contour prediction.

It is further supported by Lazzara and Porter (11) who theorise that the inward movement of the IAJ in the platform switching concept shifts the inflammatory cell infiltrate inward and away from the adjacent crestal bone, which limits the bone change that occurs around the coronal aspect (i.e. shifting inwardly the IAJ and repositioning the inflammatory cell infiltrate and confining it within a 90 degree area that is not directly adjacent to the crestal bone).

The overall effect is that the clinical use of this concept has demonstrated consistent results in relation to crestal bone preservation, which is particularly useful in aesthetic areas and also redefining the biological width surrounding dental implants.

The factors which have been linked to crestal bone loss and or preservation include:• The three-dimensional position of the placed implant and the bone circumference• The presence of an inadequate band of attached gingiva• The periodontal biotype of the patient• Microbiological leakage or micro-movement at the implant abutment interface• Contraction of graft tissues (i.e. following block chin or hip grafts) • Length of surgical procedure• The position of the polished collar (submerged versus a non-submerged approach)• Surgical flap design for surgery and exposure• Biological width• Stringent surgical techniques• A stable occlusion• Precise technical work• A motivated patient• The presence of a machined surface without retentive elements• The disruption of the soft tissue interface by utilizing healing abutments• The concept of platform switching• The bone quality in the area of implant placement• Excessive loading of crestal bone or inadequate loading of crestal bone• The proximity of implant fixtures next to each other.Current concepts that are presently used to minimise crestal bone loss are:• Deep placement of the implant fixtures ( utilized by Bicon and Ankyloss)• Platform switching (Ankyloss, 3i Prevail)• One part implant (Nobel Direct, Straumann, BioHorizons)• Scalloped design of implant fixture (Nobel Perfect)• Biologic width design implant with SLA surface (Straumann)• Biomechanical design implant with a roughened surface (Biohorizons, AstraTech).